1. Field of the Invention
This invention concerns a system for increasing conversion yields and reducing energy consumption in multiunit reactors for heterogeneous synthesis and more particularly for the synthesis of ammonia, each unit consisting of an outer shell (P), and of an internal cartridge (C) with a diameter (Dc) and total height (H1), and containing a catalyst in granules arranged in a catalytic bed (K) with axial height (H1) and situated between a cylindrical external wall with diameter (Di) and height (Hi) and an inner cylindrical wall with diameter (D'i) (smaller than Di), and height H'i, inside which the gas, the upper traverse face of the bed being open and penetrated axially by the synthesis gas.
2. Description of the Related Art:
It is well known that reactors for catalytic synthesis under pressure, and more particularly for the catalytic synthesis of ammonia, methanol, higher alcohols, etc., consist of an outer shell and of a single unit or of several units according to the C. F. Braun process, each unit having an internal cartridge containing a catalyst in granules in various shapes and with various characteristics arranged in one or more layers (catalytic beds). In reactors according to the C. F. Braun process each unit has a single-layer catalytic bed.
The synthesis gas running through the various catalytic beds is generally cooled between one bed and the next, these being exothermic reactions, in order to achieve optimal temperature conditions in the various catalytic beds by injecting fresh gas (quench reactor) or by direct interchange with the cold incoming gas. Said cooling is carried out in heat exchangers arranged outside and amongst the several units of the reactor where the C. F. Braun process is concerned.
Reactors featuring radial gas flow in the catalytic beds have recently been put forward (Lummus, Topsoe, Kellogg, U.S. Pat. Nos. 3918918 and 4181701, European Patent Appl. No. 007743-A1) or featuring axial-radial flow (Ammonia Casale U.S. Pat. Nos. 4372920 and 4405562) which represent a considerable progress when compared to axial flow reactors (including the C. F. Braun reactor), particularly when it is necessary to use large quantities of catalyst, as they reduce pressure drop in the catalytic beds and thus energy consumption. In these instances the axial gas flow requires wide reactors (low length-diameter ratio of the apparatus) with high equipment costs and high energy consumption.
Ammonia Casale U.S. Pat. Nos. 4372920 and 4405562 have made it possible greatly to simplify cartridge design in synthesis reactors, achieving an internal reactor structure easily accessible for maintenance and to load and unload the catalyst with, at the same time, low pressure drop. According to the above mentioned patents, in effect, each catalytic bed consists of a perforated cylindrical external wall, a perforated cylindrical internal wall and only one sealed end (the upper wall of the basket being, in effect, fully open); and upper unperforated portion of said internal cylindrical wall (or of both cylindrical walls), together with the section of open passage situated between the upper edges of the two cylindrical walls arranged on a level approximately perpendicular to the longitudinal axis of said perforated walls, constituting the passage where a minor portion of gas runs through the catalytic bed with a prevalently axial flow, while the remaining major portion of gas runs with radial flow through the major portion of catalytic bed arranged in the perforated area of the two cylindrical walls of the bed.
According to the above invention the portion of gas running through the bed with a prevalently axial flow is controlled by the height of the portion of unperforated internal cylindrical wall (or both internal and external) which in any case according to a prevalent characteristic of the invention is a minor portion of the total height of the wall itself. Reactors with dimensions equal to a height/diameter ratio greater than 10 (i.e., with a diameter/height ratio smaller than 0.1) are preferably to be produced according to the above invention.
In the current economic world situation modernisation of existing plants is becoming increasingly important: the majority of such plants where a synthesis process is concerned (for example of the synthesis of ammonia) uses reactors with the gas flowing axially through the catalytic beds (axial reactors) characterised by a low height/diameter ratio of the reactor, owing to the above mentioned need to contain pressure drops in the reactor. In the case of C. F. Braun reactors consisting of several units each with a low height/diameter ratio and each containing a single catalytic bed, the same applies as mentioned above not only for existing reactors but also for the new reactors planned by C. F. Braun with two or more units still according to the above criterion.
In previous Patent Applications, more particularly in European Patent Application No. 86104911.2, the Applicants have already described a system for the reduction of energy consumption in reactors for heterogeneous catalytic synthesis under pressure and more particularly in reactors with dimensions equal to a low height/diameter ratio (less than 10); it is in particular a system suitable for revamping existent axial gas flow reactors, and more particularly Kellogg reactors, used in a great number of high capacity plants (800.div.1500 t/d) built towards the end of the Seventies.
In another Patent Application the Applications described a system for revamping "Chemico" axial reactors.
All the above plants using axial reactors are characterised by the high energy consumption not critical in those years when it was available at a low cost, since the axial-type reactor is one of the reasons for high energy consumption.
While continuing their researches the Applicants found a system which can now be easily applied for the modernisation of other axial reactors and more particularly C. F. Braun reactors, whether already in existence or newly built.